The present invention relates generally to laser systems and methods that include laser beams of two different wavelengths and, more particularly, to laser systems and methods that separate a composite return signal based upon the respective wavelengths of the signals that form the composite return signal such that signals having different wavelengths can be separately processed.
A number of different applications, such as imaging or laser tracking applications, require the use of multiple laser beams. As such, these laser systems generally include two or more lasers, each of which typically emits a laser beam having a different wavelength than the wavelengths of the laser beams emitted by the other lasers. In addition to generating multiple laser beams having different wavelengths, these laser systems can also receive composite return signals that are likewise comprised of signals having different wavelengths. For example, the composite return signal may be a reflection from an object. As such, by appropriately processing the composite return signal, an image of the object can be obtained at one of the laser wavelengths.
In order to appropriately process the composite return signal, the signals having different wavelengths are typically separated. For example, to generate a proper image of an object, the signals having a particular wavelength are preferably separated from signals having other wavelengths and then analyzed. Generally, a composite signal having signals of different wavelengths is separated according to wavelength by means of wavelength selective optical filters and gratings. In this regard, the wavelength selective optical filters and gratings readily separate the composite return signal according to wavelength if the wavelengths of the signals are quite different. As the difference between the wavelengths of the signals become smaller, however, optical filters and gratings are no longer able to effectively separate the signals.
This limitation is a sizable problem since a number of applications, such as imaging applications, that utilize multiple laser sources for emitting laser beams of different wavelengths prefer that the signals differ in wavelength by only a small amount. For example, the precision imaging and targeting of targets for laser weapons requires that the wavelength of the laser beam emitted by the imaging laser be nearly the same as the wavelength of the laser beam emitted by the high energy laser (HEL) weapon to minimize the targeting error. In this regard, any differences in the wavelengths between the signals emitted by the imaging laser and the HEL can create a somewhat inaccurate or displaced image of the target with respect to the aimpoint of the HEL on target. These effects are principally due to chromatic aberrations and other differences in the atmospheric transmission of the laser beams of different wavelengths. As such, the image of the target created from the reflected signals that originated with the imaging laser may be different than and displaced from the image of the target created from the reflected signals that originated with the HEL due to differences in the atmospheric transmission, including the chromatic aberration, of the signals having different wavelengths.
For example, in instances in which the imaging laser or illuminator is a neodymium-doped yttrium aluminum garnet (Nd:YAG) laser that emits a laser beam having a wavelength of 1.06 microns and the HEL is a chemical oxygen iodine laser (COIL) device emitting a laser beam having a wavelength of 1.315 microns, the composite return signal reflected from a target can be readily separated by conventional optical filters. However, the resulting image created from the reflected laser signals having a wavelength of 1.06 microns will differ somewhat in location from the image or aimpoint of the target for the laser beam emitted by the HEL since the effects of the atmospheric transmission of the laser beam having a wavelength of 1.06 microns are significantly different than the effects of the atmospheric transmission of a laser beam having a wavelength of 1.315 microns. As such, some difficulty may arise in precisely locating and maintaining the target aimpoint due to the inaccuracies with the target image location.
In order to create more precisely aligned images, the laser beams emitted by the plurality of laser sources preferably have wavelengths that differ only slightly, such as by one or only several parts per million. However, a composite return signal comprised of signals that differ only slightly in wavelength cannot be easily separated by conventional optical filters or gratings. As such, a need exists for laser systems and associated methods that emit laser beams of slightly different wavelengths and that reliably separate the composite return signal based upon the wavelength of the signals, even in instances in which the wavelengths differ by only one or a few parts per million.
A wavelength selective laser system and associated method are therefore provided that produce laser beams having wavelengths that are only slightly different, but that permit a composite return signal to be selectively processed in accordance with the wavelengths of the signals that comprise the composite return signal. In this regard, the wavelength selective laser system and method of the present invention can process the composite return signal such that the signals having one particular wavelength are preferentially amplified relative to signals having other wavelengths. As such, the wavelength selective laser system and method of the present invention need not rely upon optical filters or gratings in order to separate the composite return signal based upon the wavelengths of the signals that form the composite return signal.
In one embodiment, the wavelength selective laser system includes first and second lasers for producing respective laser beams. Each of the first and second lasers define a nominal gain spectrum. In one advantageous embodiment, the first and second lasers are the same type of laser, such as a chemical oxygen iodine laser, so as to have the same nominal gain spectrum. However, the wavelength selective laser system also includes a magnetic field generator disposed about at least one of the lasers for altering the gain spectrum of the laser such that the wavelengths of the laser beams produced by the first and second lasers differ by a small amount, typically on the order of one or several parts per million, so that their gain spectra do not coincide. This shift in wavelength comes about due to the effect of the imposed magnetic field on the energy levels of the lasing species. As such, the first laser will emit signals having a first wavelength and the second laser will emit signals having a second wavelength. In one embodiment, the magnetic field generator includes at least one electromagnet extending around a laser cavity of one of the lasers to thereby generate a substantially uniform magnetic field within the laser cavity. While the magnetic field generator can be disposed about either laser, the magnetic field generator of one embodiment is disposed about the second laser so as to alter the gain spectrum of the second laser such that little, if any, gain is provided by the second laser for those signals produced by the first laser according to the nominal gain spectrum.
The wavelength selective laser system also includes means for directing a composite return signal to the second laser. For example, the composite return signal may be a reflection of the laser beams emitted by the first and second lasers from a target or other object. As such, the composite return signal includes signals having both the first and second wavelengths. As a result of the altered gain spectrum of the second laser, the second laser will preferentially amplify the signals having the second wavelength relative to signals having the first wavelength. In other words, the second laser will preferentially amplify the signals of the composite return signal that were originally emitted by the second laser relative to the signals of the composite return signal that were originally emitted by the first laser.
The wavelength selective laser system can also include a signal processing system for receiving and processing an output signal from the second laser which is based almost entirely on signals that were originally emitted by the second laser. In one embodiment, the signal processing system is an imaging system for constructing an image based upon the preferentially amplified signals having the second wavelength. Although the wavelength selective laser system can be utilized for a variety of applications, the wavelength selective laser system and method can be advantageously employed to image targets for laser weapons, in which instance the first laser is a high energy laser and the second laser is an illuminator.
The second laser therefore serves as both a wavelength-selective amplifier and a resonator. In this regard, the second laser includes a laser cavity about which a magnetic field generator is disposed for alterating the nominal gain spectrum of the laser cavity. According to one embodiment, the second laser can also include a first set of reflective elements disposed on opposite sides of the laser cavity for reflecting a signal through the laser cavity in order to amplify the signal according to the altered gain spectrum of the laser cavity. As such, the first set of reflective elements define the resonator cavity section-of the second laser that serves to emit a laser beam of the predetermined wavelength. According to the present invention, the second laser can also include a second set of reflective elements disposed on opposite sides of the laser cavity for reflecting a return signal through the laser cavity. As described above, the return signal is generally a composite signal including signals that were originally generated by the second laser as well as signals that were originally generated by another laser having a slightly different wavelength. As a result of the altered gain spectrum of the laser cavity, the laser cavity serves to preferentially amplify portions of the return signal and, more particularly, those portions of the return signal that have the same wavelength as the signals originally emitted by the laser cavity. As such, the second set of reflective elements define an amplification section. The preferentially amplified signals can then be provided to a signal processing system, such as for creating an image based upon the preferentially amplified signals.
The first and second sets of reflective elements define different paths through the laser cavity. In this regard, the laser cavity typically extends in a longitudinal direction. As such, the first and second sets of reflective elements are preferably disposed about different, longitudinally displaced portions of the laser cavity. As such, the laser cavity defines both a resonator section and an amplification section.
By processing a composite return signal having signals with both first and second wavelengths by preferentially amplifying the signals having the second wavelength relative to the signals having the first wavelength, the wavelength selective laser system and method of the present invention is capable of separating the return signals having the second wavelength from return signals having the first wavelength, even in instances in which the wavelengths differ by no more than 1 part per million. As such, a target or other object can be illuminated by a pair of laser beams having wavelengths that are very nearly equal and the composite return signals can be appropriately processed so as to separate those signals that are due to reflections of the first beam from those signals that are due to reflections of the second beam. By illuminating the target or other object with first and second beams having similar wavelengths, chromatic aberrations and other differences that might arise in the atmospheric transmission of laser beams having different wavelengths are significantly reduced and the image constructed based upon the portion of the composite return signal that is due to the reflection of the second laser beam also provides an accurate representation of the target or other object for signals having the first wavelength.